ProjectSummary Staphylococcus is the most common cause of community- and hospital-acquired infective endocarditis worldwide. Staphylococcal (S. aureus) endocarditis is a serious health problem in children and adults that carries high morbidity and mortality. The incidence of this disease is increasing due to healthcare advances with widespread use of intra-cardiac devices and prosthetic heart valves, and greater prevalence of valvular heart disease. One of the key limitations with antibiotic therapy for S. aureus endocarditis is poor drug penetrationintothevegetation.Theshearjetsinducedbyultrasound-mediatedcavitation(UMC)ofcirculating microbubbles (MBs) have been utilized to dissolve thrombi within vessels and the microvasculature. Fibrin affordsanattractivetargetfordevelopingtargetedMBsinthetreatmentofendocarditiswithultrasound,asthe conjugatebindingpermitsMBadherenceeveninconditionsofhighshearstress.Weproposetodevelopfibrin- targeted MBs, and evaluate the effectiveness of fibrin-targeted MBs and UMC co-administered with systemic vancomycininthetreatmentofS.aureusaorticvalveendocarditis.Afterproductionofbioconjugateandfibrin- targeted MBs, we will evaluate MB binding to the fibrin target under conditions of variable shear stress. We hypothesize that UMC of fibrin targeted MB will improve vancomycin activity against S. aureus by improving antibiotic penetration into the vegetation, and promoting vegetation clearance. Furthermore, we hope to demonstrate that the reductions in vegetation size by UMC may decrease complications including valvular regurgitation, perivalvular extension and embolization, resulting in improved outcomes including survival. We willexaminethistherapeuticapproachinanestablishedrabbitmodelofS.aureusendocarditis,randomizedto receive either transthoracic UMC application with systemic vancomycin co-administered with fibrin-targeted MBs, versus vancomycin alone. Bacterial counts (colony forming units/ml) in the treated vegetation will be quantifiedastheprimaryoutcomevariable.Residualbacterialcountsinperipheralorgansandbrainasaresult of embolic phenomena will also be quantified. Secondary outcomes of serial vegetation size, valvular regurgitation and five-day survival will be tested. Successful completion of this research could significantly impact the clinical management of this life-threatening disease. Results of these studies will be used to advancethisnon-invasivetherapeuticstrategyintoanInvestigationalNewDrugapplication.